The present invention relates generally to vehicle anti-lock brake systems and, more particularly, to a low pressure accumulator assembly for such a system wherein the accumulator is adapted to receive and temporarily store fluid during pressure reduction phases of the anti-lock control operation.
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thus causing the wheels to lock such that excessive slippage between the wheels and the road surface takes place. Wheel lockup conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, many companies have been involved in the development of anti-lock braking systems. While typically such systems are adapted to control the braking of each braked wheel of a vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. Examples of prior art anti-lock brake systems are disclosed in U.S. Pat. Nos. 3,515,440; 3,731,979; 3,870,376; and 3,880,474.
Generally, prior art anti-lock brake systems include a central control unit for monitoring the speed of the controlled wheels to determine the deceleration of the controlled wheels. When the brakes of the vehicle are applied and the wheel deceleration of the monitored wheels exceeds a predetermined deceleration threshold, indicating that there is wheel slippage and the wheels are approaching a lockup condition, the central control unit functions to control the application of hydraulic pressure through a control valve means to the associated brakes to prevent lockup of the controlled wheels. Typically, the anti-lock brake system includes means for cyclically reducing and reapplying pressure to the associated brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver. While some systems utilize a separate hydraulic pump as the means for reapplying pressure, other systems, such as disclosed in U.S. Pat. No. 4,418,966, do not require the use of a separate hydraulic pump.
In controlling the application of pressure to selected wheel brakes, many systems utilize a low pressure accumulator which is operative to temporarily receive and store brake fluid during pressure reduction phases of the anti-lock operation. The low pressure accumulator typically maintains fluid stored therein at a predetermined minimum pressure determined by a compression spring acting on an accumulator piston, generally in the range of 30-60 p.s.i. This minimum pressure represents the lowest pressure to which the controlled brakes can be reduced during anti-lock operation. However, it has been found that, in certain braking situations, it is necessary to reduce the controlled pressure below this minimum pressure in order to achieve the desired control.